This invention relates to a method of identifying fluid materials, ie analytes, and is particularly but not exclusively concerned with gaseous analytes.
It is known that some materials interact with certain gaseous analytes selectively and that this interaction can be detected from a change in the physical properties of the sensor material, for example, the mass, stiffness or, where the sensor material behaves like a liquid, viscosity. One method of detecting such change is to coat a piezo-electric component with the sensor material, drive the component to oscillate, and measure any change in the oscillation frequency. Devices which exhibit such a response, such as piezo-electric crystal oscillators and surface acoustic wave devices are referred to generically herein as chemical sensors. The change in the properties of the coated device resulting from interaction with the gaseous analyte causes gain and phase changes in the response of the piezo-electric device to applied voltage. This change may be monitored by incorporating the device in an oscillator circuit and monitoring the frequency.
The interaction effect depends upon the nature of the sensor material, ie the coating, and the gaseous analyte. Each sensor material will have a different response to different analytes according to the degree to which the sensor material absorbs, adsorbs, or otherwise interacts with, the analyte.
An important factor in this method of identifying gaseous analytes is the degree to which the interaction effect is reversible. It is important in many applications that when the sensor is withdrawn from a test material which has caused a measurement response the measurement should revert to its original value. Instead of withdrawing the sensor the analyte concentration may be reduced by a purging process, replacing the analyte carrying gas by a carrier gas (eg nitrogen). It is found that, as a general rule, the degree of reversibility varies inversely with the selectivity of the particular sensor material. Thus, if a particular sensor material responds to only a single analyte the interaction is unlikely to be reversible. Such a sensor would have a strong chemical interaction with the analyte.
The weakness and strength of interactions are dependent on thermodynamic and kinetic factors. The thermodynamic factor is to the effect that a low heat of reaction would generally indicate a weak interaction and a high heat of reaction great interaction "strength". However, in certain cases the kinetic factor may be unfavourable to reversibility despite a readily reversible thermodynamic reaction.
A strong interaction resulting from a high heat of reaction cannot of course be made weaker by unfavourable kinetic factors.
With few exceptions then, chemical selectivity for a particular analyte may only be achieved in chemical sensors at the cost of limited reversibility.
This trade-off between chemical selectivity and reversibility may be overcome by the combination of a number of chemical sensors (an array) and pattern recognition. Each sensor in the array may employ a sensor material such as to provide a weak chemical interaction with analytes of interest and therefore a reversible response, and to have a selectivity that is different, if only mildly different, from that of other members of the array. When exposed to a particular analyte the pattern of responses across the array reflects the nature of the analyte in a specific manner. Piezo-electric chemical sensors are particularly appropriate for this approach as they are low cost, easy to fabricate with different chemical selectivities and have the potential for integrating a number of sensors on a single substrate. There is nevertheless the disadvantage of having to employ a relatively complex array rather than a single sensor.